Electric brake systems typically include a mechanism for converting the rotary motion of an electric motor to a linear motion of a friction pad in the axial direction to press the friction pad against a brake disk, thereby generating a braking force. In order to control the braking force to a desired magnitude, a load sensor unit is mounted in many of such electric brake systems at a portion where a reaction force of the load applied to the friction pad is received. The largest load applied to the load sensor unit (i.e., to the friction pad) is about 30 kN. For the purpose of improving the responsiveness of braking, it is preferred that a load sensor unit be used which is capable of detecting the applied load with a minimum displacement of its movable parts.
An example of the load sensor unit capable of detecting a large load with a minimum displacement of its movable parts is disclosed in the below-identified Patent Document 1. This load sensor unit comprises a pair of opposed annular pressure plates, piezoelectric crystal elements sandwiched between both opposed pressure plates, insulating plates each electrically insulating one of the opposed pressure plates from the piezoelectric crystal element, and a lead wire through which the voltage generated by the piezoelectric crystal elements is extracted.
When an axial load is applied to the load sensor unit disclosed in Patent Document 1, the load generates a compressive stress in the piezoelectric crystal elements, and a voltage corresponding to the magnitude of the stress is thereby generated in the piezoelectric crystal elements. Therefore, by measuring the voltage generated in the piezoelectric crystal elements, the magnitude of the load can be detected. Since the displacement of the pressure plates due to the deformation of the piezoelectric crystal elements is minute, the responsiveness of the braking is not compromised when the sensor unit is mounted in to the electric brake system.
However, since this load sensor unit is designed such that the piezoelectric crystal elements directly receive the load applied, there is a possibility that one or more of the piezoelectric crystal elements may chip or crack when an impact load or a load oblique to the axial direction is applied thereto. Further, since the applied load also acts on the insulating plates each electrically insulating one of the opposed pressure plates from the piezoelectric element, high durability is required for the insulating plates. It is therefore difficult to use an inexpensive insulating material such as resin for the insulating plate because such an inexpensive insulating material is insufficient in durability.
In view of this, the present inventors have investigated for a load sensor unit which is capable of detecting a large load with a minimum displacement of its movable Parts and which is excellent in durability, and have developed in-house a load sensor unit shown in FIG. 15.
A load sensor unit 80 shown in FIG. 15 comprises a flange member 2, a support member 3, a magnetic target 4, and a magnetic sensor 5. The flange member 2 is supported by the support member 3 from the axially rearward side, at a position offset radially outwardly from the input position of the load, so that the flange member is deflected when a load is applied from the axially forward side. The magnetic target 4 is fixed to the flange member 2. The magnetic sensor 5 is fixed to the support member 3 so that it can detect a magnetic flux generated by the magnetic target 4.
When a load is applied to the flange member 2 of the load sensor unit 80 from the axially forward side, the magnetic target 4 and the magnetic sensor 5 move relative to each other due to the deflection of the flange member 2. This relative displacement between the magnetic target 4 and the magnetic sensor 5 causes a change in the output signal of the magnetic sensor 5, corresponding to the magnitude of the relative displacement. As a result, the magnitude of the load can be detected based on the output signal of the magnetic sensor 5. With this arrangement, when a load is applied to the load sensor unit 80, the load acts on the flange member 2 to cause deflection in the flange member 2, but it does not act on the magnetic sensor 5. Therefore, the load sensor unit is less likely to malfunction even if an impact load or a load oblique to the axial direction is applied, thereby ensuring a high durability.